Where lands within a proposed or existing irrigation district are owned by co-owners only, such co-owners who are residents of this state may vote their respective interest personally, or such co-owners by an instrument in writing, may designate one of their number as agent to cast the vote for each such co-owner. Provided, that in no event shall any such co-owner be entitled to cast, less than one full vote, or any vote or votes constituting any fraction of one vote. Such instrument shall be acknowledged by such co-owners and shall be presented to and filed with the clerk of the district election board.